The operation of a control valve involves positioning a plug relative to a stationary seat within the valve, whereby an actuator that is directly coupled to the valve plug via a stem is used to move the valve plug to the desired control position. The action of the valve stem can be either linear or rotary, depending on whether the valve is a liner or rotary valve.
Typically, for an automatically controlled valve, a pneumatic or electric actuator is used to manipulate the valve stem. In either case, the actuator is operated by a control device such as a computer, controller, PLC, thermostat, and/or other electrical controlling device, all of which are referred to herein generically as a “controller,” that sends an electrical control signal to an electro-mechanical or electro-pneumatic device that operates the actuator.
By automating a valve in this manner, the valve can be located in a remote, dangerous, flammable, and/or toxic environment, and/or in a location that is difficult to reach. Such valves are often used to directly and/or indirectly control temperatures, pressures, and flows within an open or closed-loop system. Heat exchangers are a common type of closed-loop control application where both pneumatically and electrically actuated control valves can be used to regulate water, steam and condensate.
Regulation of an automatically controlled valve can sometimes be implemented by monitoring the flow rate through the valve and/or other process parameters, and adjusting the valve position until the desired process parameters are achieved. However, as modern systems have continued to demand higher efficiencies, the need has increased for improved control valve performance, often demanding fast response and accuracies within 2% of set point. One approach is to include a valve positioner as part of the control system. A valve positioner is a device that provides direct feedback to the valve controller as to the positioning of the valve stem. This allows the controller to accurately control the valve, and to correct for errors due for example to mechanical wear or dimensional changes in mechanical linkages caused by temperature and pressure fluctuations.
Generally, a valve positioner includes a valve coupler that is mechanically linked to the valve stem or actuator, and is sensitive to the position of the stem. The valve positioner is coupled to a sensor that senses the position and/or movements of the valve coupler and thereby determines the position of the valve stem, and an electronic interface that transmits information regarding the valve stem position to the valve controller.
Some of the advantages of using valve positioners include:                faster speed of response, as compared to monitoring process parameters;        accurate control when high and/or varying differential pressures across the valve plug cause the valve position to change;        facilitation of control action change: while the use of a positioner cannot change a valve's function, it can switch its control action, for example from direct to reverse and vise-versa;        change of a control valve's flow characteristic, such as linearization of a non-linear flow characteristic by switching a mechanical cam or by digitally or electronically reprogramming a new performance curve into the positioner;        improved safety, by ensuring that a valve does not travel beyond its design/safety limits; and        improved digital communications and diagnostics capability when the positioner is interfaced, e.g. to a control systems that utilizes digital communications such as HART, BACnet, Modbus or LonWorks protocols.        
While the feedback provided by a valve positioner leads to many benefits, the accuracy of the valve position sensing can be limited by backlash in the mechanical valve coupler of the valve positioner. Accordingly, valve couplers sometimes include an anti-backlash feature that maintains the moving parts of the valve coupler under tension, so that backlash is avoided.
In addition, the valve coupler is typically operable only over a limited range of motion, such that it is often desirable to be able to adapt the range of motion of the valve coupler to the configuration of the valve stem or actuator. However, alignment of the valve coupler can lead to misalignment of the sensor and/or incorrect tensioning of the anti-backlash feature. For this reason, it is often desirable to interpose a friction clutch between the valve coupler and the sensor and/or anti-backlash feature.
FIG. 1A is an exploded view of a prior art valve coupler that includes a rotatable shaft 10 that frictionally engaged at its distal end to a gear 12 which is operable over a limited range of about 75 degrees of rotation. The frictional engagement is by means of a clutch 14 formed as a stack of washers 14 that are pressed together. The gear 12 engages with a mechanically driven sensor (not shown) that is in electronic communication with the electronic interface (not shown).
A helical torsion spring 16 is provided as an anti-backlash feature. A proximal end 18 of the spring 16 is fixed to the housing, and a distal end of the spring 20 is fixed to the gear 12. Accordingly, the rotational position of the shaft 10 can be adjusted by frictional sliding of the clutch 14, while the tensioning of the spring 16 remains unaffected. If an attempt should be made to rotate the shaft 10 beyond its operating limits, the clutch 14 will function to protect the gear 12, sensor, and spring 16 from damage by allowing the shaft 10 to rotate while the gear 12 remains fixed.
Since automated valves that utilize valve positioners are often located in harsh, dirty, corrosive, flammable, and/or toxic environments, it can be desirable to surround the electronic interface of the positioner by a sealed enclosure, so that the interface is not subjected to environmental attack, and so that flammable vapors in the surrounding environment cannot reach the electronics of the interface, where they might be ignited by an electrical spark or other electronic activity of the interface. The valve coupler 10 of FIG. 1A penetrates the housing (not shown) at the indicated plane 22, so that the gear 12, clutch rings 14, and spring 16 are within the housing. A seal (not shown) is provided that allows the shaft 10 to enter the interface enclosure while excluding leakage of surrounding gasses and vapors into the enclosure.
However, this penetration of the interface enclosure and need for a seal increases the maintenance requirements of the positioner, and poses a danger of leakage of gasses past the seal, whereby the electronic interface might be corroded and damaged, and/or ambient flammable vapors might be ignited.
One approach is to use a magnetic coupling between the valve coupler and the sensor, such that the position and/or movements of the valve coupler are sensed magnetically, without requiring physical penetration of the interface housing. However, due to the limited space that is available in the region of the valve coupler outside of the interface enclosure, this approach typically does not allow for implementation of an anti-backlash feature and/or clutch.
What is needed, therefore, is a valve positioner that includes an anti-backlash feature and clutch, and yet does not require penetration of the sealed interface enclosure by the valve coupler.